Dampers, or shock absorbers, are well known and widely applied in, for instance, a variety of vehicles like cars, truck, buses and trains. The dampers are designed to provide a desired damping behavior between parts moving with respect to one another. Damping can be chosen to be stiff or soft by the specific design of the damper. More advanced dampers have been proposed, which provide a frequency selective damping behavior to the damper. A frequency selective valve can be added to the damper to provide a desired frequency selective damping behavior. Such frequency selective valve can be added to or incorporated into known damper configurations to provide additional frequency selectivity. On the other hand, such frequency selective dampers can also be employed in their own right in certain applications that require frequency selective damping of a fluid flow between two (pressure) chambers.
A pressure or control chamber can be employed in frequency selective valves and dampers having such type of valve incorporated. Upon a fluid flow in the valve which is to be damped so as to provide the damping behavior, a part of the fluid flow can be branched off to increase a pressure inside the pressure or control chamber. An increase in pressure inside the control chamber then acts to increase a closing force on a controlled valve provided in the flow channel for the fluid flow that is to be damped. The closing force of the controlled valve controls the momentary damping behavior.
One likes to have a predetermined increase of the closing force as a function of time, such as, for instance, a proportional relation between closing force and time. However, a desired relation between closing force and time is not easily obtained, if at all, in the presently known configurations of frequency selective valves. The increase in closing force generally shows a strong non-linear dependence on the pressure increase in the control chamber and therefore a strong non-linear dependence of time. One would like to have various parameters available to tune the closing force as a function of pressure inside the control chamber and thus as a function of time.
A pressure increases in time inside the control chamber to control the closing force of the controlled valve so as to provide frequency selective damping. However, for the known configurations the pressure inside the control chamber does not go back again to a neutral level for a next fluid flow to be damped, such as during a next stroke of a piston in its cylinder, which is to be damped. Having the control chamber pressure maintained at some level above neutral strongly deteriorates the performance of the frequency selective valve.
The presently known configurations further may show a strong dependence of individually manufactured valves on manufacturing tolerances. There is a need for a frequency selective valve configurations that is very robust to manufacturing tolerances so that any desired damping behavior will actually be achieved in any valve produced.
EP 1 442 227 A1 of applicant discloses a frequency dependent damper valve according to the field of the invention. Such damper valve proves to be very advantageous in, for instance, shock absorbers for cars to obtain a different damping behavior for movements associated with the car body and movements associated with the car wheels. The damper has a membrane as a flexible wall of the control chamber in contact with the movable valve body, which provides for an effective surface onto which the pressure in the control chamber acts to exert a closing force onto the controlled valve. However, the effective surface changes rapidly with movement of the movable valve member, which results in a non-linear dependency of the closing force of the controlled valve with the pressure inside the control chamber, an increase in closing force decreasing with increasing pressure in the control chamber. Further, it requires quite some force to deform the membrane, which will make dependence of the closing force of the controlled valve on the pressure inside the control chamber even more non-linear. The membrane becomes very stiff and provides a small effective surface at large deformation upon movement of the movable valve body outward of the control chamber. This provides for a non-optimal damping behavior, and makes the damping behavior of the valve very dependent on manufacturing tolerances. Generally, a pack of controlled valve plates is mounted on the movable valve member. The total height of the assembly will vary due to manufacturing tolerances, which has a very large effect on the performance of the damper valve. In some instances the pressure in the control chamber is not sufficient to provide enough closing force onto the controlled valve. It turns out that a large percentage of damper valve manufactured has to be discarded as their damping characteristics do not comply with the required specifications.